Arctic sea ice has reached the second lowest extent ever recorded, according to the US national snow and ice centre, and a new map shows how far the 2008 melt has receded compared to the historical average.

The map, produced by Collins, illustrates that the area of ice that is at least five years old has fallen by more than half since 1985. It comes as the Northwest Passage, above north America, and the Northeast Passage, over the top of Russia, are both free of ice for the first time.

"While slightly above the record-low minimum set last year, this season further reinforces the strong negative trend in summer sea ice extent observed during the past 30 years. Before last year, the previous record low for September was set in 2005", said a spokesman for the Snow and Ice center based at the University of Colorado.

A spokesman for WWF International said: "This means two years in a row of record lows. The trend of melting Arctic sea ice is alarming for the rest of the world. The Arctic is a key factor in stabilising the global climate so this is a global problem that demands an immediate and global response."

The daily rate of ice loss usually starts to slow in August as the Arctic begins to cool. But in August 2008, the daily decline rate remained steadily downward and strong.

The average daily ice loss rate for August 2008 was 78,000 square kilometers (30,000 square miles) per day. This was the fastest rate of daily ice loss that scientists had observed since satellite photographs were started in 1979.